package LeetCode.LC106;

/**
 * @author corn
 * @create 2020-08-06-23:05
 */

/**
 * 106. 从中序与后序遍历序列构造二叉树
 * 根据一棵树的中序遍历与后序遍历构造二叉树。
 * 注意:
 * 你可以假设树中没有重复的元素。
 * 例如，给出
 * 中序遍历 inorder = [9,3,15,20,7]
 * 后序遍历 postorder = [9,15,7,20,3]
 */
public class BuildTree {

}
class Solution {
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        return helper(inorder, postorder, postorder.length - 1, 0, inorder.length - 1);
    }

    public TreeNode helper(int[] inorder, int[] postorder, int postEnd, int inStart, int inEnd) {
        if (inStart > inEnd) {
            return null;
        }
        //找到当前根结点的值
        int currentVal = postorder[postEnd];
        //找到当前根结点
        TreeNode current_root = new TreeNode(currentVal);
        //在中序遍历中找到currentVal的下标
        int inIndex = 0;
        for (int i = inStart; i <= inEnd; i++) {
            if (inorder[i] == currentVal) {
                inIndex = i;
            }
        }
        //整理左右子树
        TreeNode left = helper(inorder, postorder, postEnd - (inEnd- inIndex) - 1, inStart, inIndex - 1);
        TreeNode right = helper(inorder, postorder, postEnd - 1, inIndex + 1, inEnd);
        //连接左右子树
        current_root.left = left;
        current_root.right = right;
        //返回根结点
        return current_root;
    }
}
class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    public TreeNode(int val) {
        this.val = val;
    }
}
